For a long time now the infrastructure represented by telecommunication networks has been growing rapidly while at the same time these networks have become more versatile and technologically more advanced. The old fixed analog telephone network has been replaced by a digital one. Mobile networks have been set up to complement the fixed telephone network, enabling the use of wireless terminals with little geographical limitations. On the other hand, separate data networks have been built. These include a great number of local area networks (LAN) of private nature, and a plurality of public data networks. The most important of the public data networks is the global Internet through which a vast number of servers and other sites can be reached. One of the factors that make it so tempting to use the Internet for a variety of data transfer purposes, such as to transfer voice and picture signals, is that it is so inexpensive, i.e. the cost payable by the user per amount of data transferred is relatively small.
At the core of the so-called system architecture of the Internet there is a protocol pair called the TCP/IP (Transmission Control Protocol/Internet Protocol). Transmission session is controlled at the transfer end points according to the TCP, and TCP data frames are fragmented into smaller “packets” and these packets are then routed through the network according to the IP. Routing is generally based on network addresses of terminals, these addresses being global IP addresses in the case of the Internet. On the basis of a plain-text receiver address given by the user, the sending terminal is able to get a corresponding IP address from a server set up especially for this purpose and to attach that IP address to the data packets to be sent. If the recipient is at a usual location such as home or office, no problems usually occur with the transfer. The situation becomes more complicated if the recipient is moving. Even if he were close to a machine interfaced with the Internet, the transfer could not be completed if the sender did not know the network address for the machine in question. The sender cannot even know whether the recipient can be reached or not. It is conceivable that the sender first calls the recipient on his mobile phone to inquire about this, but this is a clumsy way and, moreover, incurs phone costs. Furthermore, the recipient's mobile phone may be switched off or the recipient may not have a mobile phone to begin with.
Publication WO 9816045 discloses a system which alleviates the problem mentioned above. FIG. 1 illustrates the overall structure of the system. It includes a server 120 interfaced with a data network 110 such as the Internet, and terminals, normally computers, interfaced with that same network. FIG. 1 shows two terminal computers DTA and DTB. A system user reports to the server at a terminal computer. A registration message contains the network address of the terminal computer in question and the password and personal identifier entered by the user. The server responds by sending a list of persons who at that moment can be reached so-called “OnLine.” The list only includes persons belonging to a group defined by the user. In FIG. 1, a user, person A, is sitting at terminal computer DTA, and a user, or person B, is sitting at terminal computer DTB. If person A wants to contact person B, he sends a message from terminal computer DTA to the server 120, which message contains the name of person B. If person B can be reached, the server responds by sending to terminal computer DTA, via a route 111, the network address ADB of terminal computer DTB. In addition, the server sends to terminal computer DTB, via a route 112, the network address ADA of terminal computer DTA and the name of person A. After that, persons A and B can begin to communicate via a route 113, where the server is no longer needed. A person who has joined this kind of a service may also reserve a right to restrict the distribution of his address information. In that case the server that received a connection request for such a person first asks the person in question whether the requester can be sent information about whether the person can be reached. The person can then either allow or deny the sending of that information.
So the idea of the system disclosed in publication WO 9816045 is to make it easier for persons belonging to the same circle of acquaintances, for example, to establish connections between them. To that end, the server keeps a register for the group associations, personal data and online traffic data of the persons subscribing to the service. A drawback of the system is that a user must himself start, at the terminal computer, a software application according to the system and enter his personal identifier so as to register in on the server. Moreover, the user must actively observe his computer so as to be able to respond to a connection request. Another drawback is that communication is limited to the exchange of text messages and files, and the user is tied to the terminal computer for the duration of the connection.
Publication WO 0165821 discloses a system employing a data network, involving portable devices such as laptop computers in the vicinity of network terminals. A portable device is equipped with a small-range radio device and software required by the system in question. The portable device repeatedly sends out a reporting message. When it comes close to a data network terminal, the latter detects the reporting message and sends to a data network server a registration request containing user-specific information that was included in the reporting message. This way the user's device is linked to the network. The system does not include any special connection establishing service.